Let me answer to the million dollar question here:
"Why is Dilraba playing Changge, a character that's still in her teens?"

A counter question in disguise of an answer for you guys:
1. "Why does Yangmi play Fuyao in Legend of Fuyao? Why does Zhao Liying play Fei in Legend of Fei and Chu Qiao in Princess Agents? Why Li Qin in The Wolf? Why Peng Xiaoran in Goodbye, my Princess? Why Tiffany in Princess Weiyoung and Legend of Xiao Chuo?"

2. " Why Xiao Zhan, as Tang San in Duoluo Continent? Why Darren in The Wolf? Why Vengo Gao as Dijun in Eternal love of Dream? Why Mark as Ye Hua? Why Kenny Lin as Yuwen Yue?"
 
And the list goes on...
Time for answers:
Because, in one word, they all had/have what it takes to play these roles that no other person could replace them as. They are unique choices to be cast as these roles because of their Charisma.
And lastly why Dilraba as Changge Xing?
1. The director : " I think the manhua artist drew the illustration looking at Raba. "
2. The screenwriter ( I'm not sure who she is, might also be the producer) : 
"Raba is destined to play this role for the mole under her eye."

So its just not me, who thinks Dilraba is perfect for the role of Changge. The director, screenwriter/ producer themselves think so and she was the only one they ever considered for the role

Now just because Dilraba is in her late twenties (But still looks like she's in her late teens/early twenties, except when in mature lady make-over), should NEVER be stopped to be cast, right? 

3. She has what it takes, she meets all criteria, she has the charisma and the Director and Writer find her perfect for the role.

 But people choose to still bring her age into consideration in order to bring her down? Why?
Point to be noted:The drama adaptation never specified their age either. Its just the assumption from the manhua. But if it did specify her age to be 15. What's wrong with that? When there's absolutely nothing wrong when Xiao Zhan plays Tang San and Darren plays the Wolf and Kenny Lin plays Yuwen Yue and Zhao Liying plays Fei and Yangmi plays Fuyao?

Can you guys stop bringing age into discussion over and over again? This is a historical piece, not a highschool drama that needs cast with appropriate age to play highschool students, does it? These kinds of series always require Charismatic actors who can slay the role. Isn't that exactly what Dilraba and the others in the Long Ballad doing ? 

  1. Dilraba is in her late 20th like you said, her youth is still remarkably well preserved. The young ChengGe is supposedly mentally mature than her actual age in the manga judging with her role of leading Army and an expert in war strategic. I don't see her age is a problem. It's not like drama the rebel princess 40 something woman played 15 years old princess. 

Agreed. I honestly don’t see why its such a big deal that Reba is playing Changge, she fits and acts the role well so I don’t see what the problem is. 

Another thing that you pointed out: their age isn’t mentioned in the drama whatsoever. From the drama alone, I’d put Changge to be around 17-18 years of age from her actions and ideas. She still has that naïveté in wanting to seek out vengeance for her family at the expense of her own life, but then there are other scenes where you can see how mature she is. 

I don’t think they made a mistake in the casting at all, sure Reba might be older than what her character is supposed to be; same for Leo, but at the end of the day, what matters is that they play their character well and that’s exactly what they’re doing. 

Exactly. Like I said their abilities and images matter the most. Dilraba is pulling off as Changge brilliantly, in my opinion. Also Wu Lei is doing an amazing job as Ashina Sun.

Couldn't have agreed more with all you! I, too, noticed that they didn't really mention the ages. This drama seems to have an element of fulfilling visual fantasies, there were quite a few scenes where I thought they evoked a real life version of 'manhua' or anime. Like when her hair was blowing in the wind, and that snow scene, just to name a few. I was wowed by how visually appealing it looked found it quite satisfying. 

She's one of the first Chinese actresses that come to mind when I think of a strong female/warrior. Some other ones would be Li Xin, Bai Lu, and Liu YiFei. 

Leo Wu may look a bit young, I keep saying that it's probably also due to his smaller face, but he carried his character well and looks really good in the tribal get-up and hairstyle. He comes off as this young man who had to grow up fast and shoulder more responsibilities, and suits the look of the Eagle faction. Actually, all 3 factions look their part, the bear, the wolf and the eagle. Plus Great Khan looks the part tremendously. 

I do think that, ideally, the casting could have been even better, but this is pretty good for me.

I agree with the visually appealing part. So many scenes were visually appealing and whoever designed Wu Lei's tribal costumes deserves an award!

I love Dilraba in this role.  

@Dragnelia, Yes! I loved his darker costume, it made him look even more like the character as well as more masculine and intimidating. He really knows how to act through his eyes, I find myself rewinding parts just to enjoy those scenes again, lots of depth to them.


@Kim MC, Me too!

@Alexis Romanov He nailed it completely. He has become my favorite now. I'm loving Wu Lei's acting and grasslands costumes so much. His expressions are on point. 

@Kim MC Dilraba, too, looks more gorgeous in grassland costumes. That blue dress ? Can't get over her in that. Her beautiful eyes. I'll be heartbroken to see them back in Tang costumes again xD

A select few actors are blessed with an "it" factor and have the ability to be absolutely  riveting onscreen reading toilet paper ingredients.  Dilraba is one of those actors / actresses blessed with an abundance of charisma but has had the misfortune of starring in dramas cursed with writing mediocrity.  From episode 1 of "The Long Ballad", she has embodied and beautifully acted the character of Li Chang Ge and I cannot get enough of the storyline and actor portrayals.  

I also think it needs to be noted that very few young actors and actresses would have the acting skills and talent to pull off the deep emotions and struggles that these characters have to go through, especially in the case of Changge. She has a very mature personality that perhaps a younger actress would not be able to portray well.

@Kim MC I couldn't agree more, in her previous projects it was her charisma that lured in audiences even though the writing wasn't great. And finally, she is dazzling like a star in greatly written character and story in The Long Ballad. I've always thought that she never got roles that let her shine and bring the best out of her. Finally, she got it and she's so happy about it. 

@SerenityShimikaBlack Yes. This fact is a significant point. The characters here has such depth of emotions, so much complexity and conflicting feelings. I could never substitute Dilraba or Wu Lei with a better fit. Not just them. I think each and every character here plays their part excellently.

I agree with every point you have made!! People who keep bringing up her age are just classic misogynist and ageist. Age should never be a barrier when playing a role because that's what it is - a performative narrative. She has the ability to convey the inner turmoil of a broken young woman whose entire world has collapsed. Changge is such a layered character whose character appears as one dimensional with her strengths being confident, sometimes arrogant, brave. But we see throughout the series that these strengths are also flawed and like any developing woman, is learning to accept those truths. She plays these character traits and development so well. Aside from the physical resemblance between the manhua and Dilraba, it would've been disrespectful to the source material to not have an Uyghur actress to play the character.

@Martial I think so too. Also she has emphasized in each of her interview how she feels the most connected to this character out of all that she played and instantly could get into character without effort. And about the misogynist and ageist fact, I couldn't emphasize it more. Look at how people never pointed out how older Vengo or Vic Chou was when she was way younger than now while playing ELOD and Flame's Daughter. Just because she's matured a bit in age now, she has become a hot topic to gossip against. But now that more people are appreciating their work in The Long Ballad and also getting dazzled in Wu Lei's and Raba's chemistry, I feel overjoyed. 

 Dragnelia:

@Martial I think so too. Also she has emphasized in each of her interview how she feels the most connected to this character out of all that she played and instantly could get into character without effort. And about the misogynist and ageist fact, I couldn't emphasize it more. Look at how people never pointed out how older Vengo or Vic Chou was when she was way younger than now while playing ELOD and Flame's Daughter. Just because she's matured a bit in age now, she has become a hot topic to gossip against. But now that more people are appreciating their work in The Long Ballad and also getting dazzled in Wu Lei's and Raba's chemistry, I feel overjoyed. 

Absolutely. They never point out that when producers and casting directors are always casting younger actresses to play alongside much mature actors. But the minute, an actress has advanced in age, it's almost as if the talent has left the body to a lot of these critics. I see why a lot of male actors say their best works come later in life because regardless how old they get, the roles being offered to them will always be complex characters whereas viewers expect actresses to be reduced to the same confining roles. I love Dilraba as an actress and I am overjoyed that she is beginning to pick the roles she wants to portray.